FM Transmitter
6 FM TRANSMITTER Introduction There is too much over-crowding in the AM broadcast bands and shrinkage in the nighttime service area due to fading, interference, etc. FM broadcasting offers several advantages over AM such as uniform day and night coverage, good quality listening and suppression of noise, interference, etc. All India Radio has gone in for FM broadcasting using modern FM transmitters incorporating state-of-art technology. The configurations of the transmitters being used in the network are : 3 kW Transmitter 2 x 3 kW Transmitter 5 kW Transmitter 2 x 5 kW Transmitter
Salient Features of BEL/GCEL FM Transmitters 1. 2. 3. 4. 5. 6. 7. 8.
Completely solid state. Forced air cooled with the help of rack-integrated blowers. Parallel operation of two transmitters in passive exciter standby mode. Mono or stereo broadcasting Additional information such as SCA signals and radio traffic signals (RDS) can also be transmitted. Local/Remote operation Each transmitter has been provided with a separate power supply. Transmitter frequency is crystal controlled and can be set in steps of 10 kHz using a synthesizer.
Modern FM Transmitter Simplified block diagram of a Modern FM Transmitter is given in Fig.1. The left and right channel of audio signal are fed to stereo coder for stereo encoding. This stereo encoded signal or mono signal (either left or right channel audio) is fed to VHF oscillator and modulator. The FM modulated output is amplified by a wide band power amplifier and then fed to Antenna for transmission.
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Induction Course(Radio) Voltage controlled oscillator (VCO) is used as VHF oscillator and modulator. To stabilize its frequency a portion of FM modulated signal is fed to a programmable divider, which divides the frequency by a factor ‘N’ to get 10 kHz frequency at the input of a phase and frequency comparator (phase detector). The factor ‘N’ is automatically selected when we set the station carrier frequency. The other input of phase detector is a reference signal of 10 kHz generated by a crystal oscillator of 10 MHz and divided by a divider (1/1000). The output of phase detector is an error voltage, which is fed to VCO for correction of its frequency through rectifier and low pass filter.
Fig. 1 Block Diagram of Modern FM Transmitter
2 x 3 kW FM Transmitter Simplified block diagram of a 2 x 3 kW FM transmitter is shown in Fig.2. 2 x 3 kW Transmitter setup, which is more common, consists of two 3 kW transmitters, designated as transmitters A and B, whose output powers are combined with the help of a combining unit. Maximum of two transmitters can be housed in a single rack along with two Exciter units. Transmitter A is provided with a switch-on-control unit (GS 033A1) which, with the help of the Adapter plug-in-unit (KA 033A1), also ensures the parallel operation of transmitter B. Combining unit is housed in a separate rack.
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Fig.2 Block Diagram Of 2x3 Kw Fm Transmitter Ref.Drg.No.:-STI(T)395,(DC147) Low-level modulation of VHF oscillator is carried out at the carrier frequency in the Exciter type SU 115. The carrier frequency can be selected in 10 kHz steps with the help of BCD switches in the synthesizer. The exciter drives four 1.5 kW VHF amplifier, which is a basic module in the transmitter. Two such amplifiers are connected in parallel to get 3 kW power. The transmitter is forced air-cooled with the help of a blower. A standby blower has also been provided which is automatically selected when the preselected blower fails. Both the blowers can be run if the ambient temperature exceeds 40oC. Power stages are protected against mismatch (VSWR > 1.5) or excessive heat sink temperature by automatic reduction of power with the help of control circuit. Electronic voltage regulator has not been provided for the DC supplies of power amplifiers but a more efficient system of stabilization in the AC side has been provided. This is known as AC-switch over. Transmitter operates in the passive exciter standby mode with help of switch-on-control unit. When the pre-selected exciter fails, standby exciter is automatically selected. Reverse switch over, however, is not possible. A simplified block diagram of a 2 x 5 kW FM Transmitter is also given in Fig. 3.
Fig.3 RF Block Schematic of 2x5 kW FM Transmitter Ref.Drg.No.:-STI(T)557,(DC309)
Exciter (SU 115) The Exciter (SU115) is, basically, a self-contained full-fledged low power FM Transmitter. It has the capability of transmitting mono or stereo signals as well as additional STI(T) Publication
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Induction Course(Radio) information such as traffic radio, SCA (Subsidiary Channel Authorisation) and RDS (Radio Data System) signals. It can give three output powers of 30 mW, 1 W or 10 W by means of internal links and switches. The output power is stabilized and is not affected by mismatch (VSWR > 1.5), temperature and AC supply fluctuations. Power of the transmitter is automatically reduced in the event of mismatch. The 10 W output stage is a separate module that can be inserted between 1 W stage and the low pass harmonics filter. This stage is fed from a switching power supply which also handles part of the RF output power control and the AC supply stabilizations. In AIR set up this 10 W unit is included as an integral part of the Exciter. This unit processes the incoming audio signals both for mono and stereo transmissions. In case of stereo transmission, the incoming L and R channel signals are processed in the stereo coder circuit to yield a stereo base band signal with 19 kHz pilot tone for modulating the carrier signal. It also has a multiplexer wherein the coded RDS and SCA signals are multiplexed with the normal stereo signal on the modulating base band. The encoders for RDS and SCA applications are external to the transmitter and have to be provided separately as and when needed.
Frequency Generation, Control and Modulation The transmitter frequency is generated and carrier is modulated in the Synthesiser module within the Exciter. The carrier frequency is stabilized with reference to the 10 MHz frequency from a crystal oscillator using PLL and programmable dividers. The operating frequency of the transmitter can be selected internally by means of BCD switches or externally by remote control. The output of these switches generates the desired number by which the programmable divider should divide the VCO frequency (which lies between 87.5 to 108 MHz) to get a 10 kHz signal to be compared with the reference frequency. The stablised carrier frequency is modulated with the modulating base band consisting of the audio (mono and stereo), RDS and SCA signals. The Varactor diodes are used in the synthesizer to generate as well as modulate the carrier frequency.
Switch-ON Control Unit (Type GS 033 A1) The switch-on-control unit can be termed as the “brain” and controls the working of the transmitter ‘A’. It performs the following main functions: 1. It controls the switching ON and OFF sequence of RF power amplifiers, rack blower and RF carrier enable in the exciter. 2. Indicates the switching and the operating status of the system through LEDs. 3. Provides automatic switch over operation of the exciter in the passive exciter standby mode in which either of the two exciters can be selected to operate as the main unit. 4. It provides a reference voltage source for the output regulators in the RF amplifiers. 5. It is used for adjusting the output power of the transmitter.
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FM Transmitter 6. It evaluates the fault signals provided by individual units and generates an overall sum fault signal which is indicated by an LED on the front panel. The fault is also stored in the defective unit and displayed on its front panel.
Adapter Unit (KA 033A1) Adapter Unit is a passive unit which controls transmitter B for its parallel operation with transmitter A in active standby mode. The control signals from the Switch-on control unit are extended to transmitter B via this Adapter unit. If this unit is not in position the transmitter B can not be energized.
1.5 kW VHF Amplifier (VU 315) This amplifier is the basic power module in the transmitter. It has a broad band design so that no tuning is required for operation over the entire FM Broadcast band. RF power transistors of its output stages are of plug in type which are easy to replace and no adjustments are required after replacement. Each power amplifier gives an output of 1.5 kW. Depending on the required configuration of the transmitter, output of several such amplifiers is combined to get the desired output power of the transmitter. For instance, for a 3 kW set-up two power amplifiers are used whereas for a 2 x 3 kW set-up, 4 such amplifiers are needed. The simplified block diagram of 1.5 kW Power Amplifier is given in Fig. 4.
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Fig. 4 Block Diagram of 1.5 kW Amplifier VU 315 Ref. Drg.No.:-STI(T)444(DC196) This amplifier requires an input power of 2.5 to 3 W and consists of a driver stage (output 30 W) followed by a pre-amplifier stage (120 W). The amplification from 120 W to 1500 W in the final stage is achieved with the help of eight 200 W stages. Each 200 W stage consists of two output transistors (TP 9383, SD1460 or FM 150) operating in parallel. These RF transistors operate in wide band Class C mode and are fitted to the PCB by means of large gold plated spring contacts to obviate the need for soldering. The output of all these stages is combined via coupling networks to give the final output of 1.5 kW. A monitor in each amplifier controls the power of the driver stage depending on the reference voltage produced by the switch-on-control unit. Since this reference voltage is the same for all the VHF amplifiers being used, all of them will have the same output power. Each amplifier has a meter for indicating the forward and reflected voltages and transistor currents. Also a fault is signaled if the heat sink temperature or the VSWR exceed the prescribed limits. In both cases, the amplifier power is automatically reduced to protect the transistors.
Power Supply System
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FM Transmitter The FM transmitter requires 3-phase power connection though all the circuits, except the power amplifiers, need only single phase supply for their operation. An AVR of 50 kVA capacity has been provided for this purpose. Power consumption of the transmitters under various configurations is as follows : Frequency of operation 87.5 to 100 MHz 100 to 108 MHz
Power Consumption 5 kW 2 x 3 kW 8500 W 10200 W 8860 W 10640 W
3 kW 5100 W 5320 W
2 x 5 kW 17000 W 17720 W
These figures do not include the power consumption of blowers which is 200 W for each blower. For each transmitter, there is a separate power distribution panel (mounted on the lower portion on the front of the rack). Both the distribution panels A&B are identical except for the difference that the LEDs, fuses and relays pertaining to switching circuit of blowers and absorber are mounted on the ‘A’ panel. Power Reduction in case of Amplifier or Transistor Failure When an amplifier module or a push-pull output stage in an amplifier module fails due to failure of any one transistor, the output gets reduced according to the following formula. : Po { (m-n)/m}2
P
=
Po P M
= = =
N
=
nominal power reduced power available at the antenna total number of amplifier modules or of push Pull output stages in circuit number of faulty amplifiers or push pull output stages.
Where
The power consumed in the absorber resistors is calculated according to the formula : Pabsorber =
Po – Pn
Where Pn is the faulty partial power, which in case of failure of an entire amplifier module equals 1250 W. If power reduction occurs due to failure of one or more VHF amplifiers, the transmitter should be switched off immediately and the working transmitter should be selected on the antenna using the U-links on the Combining unit.
FM Antenna and Feeder Cable System The Antenna system for FM Transmitters consists of 3 main sub-systems, namely : a) b) c)
Supporting tower Main antenna Feeder Cable
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Tower A tower of good height is required for mounting the FM antenna since the coverage of the transmitter is proportional to the height of the tower. For a 100 m height, the coverage is about 60 km. Wherever new towers were to be provided, generally they are of 100 m height since beyond this height, there is steep rise in their prices because of excessive wind load on the top of the tower. At some places existing towers of Doordarshan have also been utilized for mounting the FM antenna. Provision has also been made on the AIR towers for top mounting of TV antenna below FM antenna (Aperture for Band III).
Antenna The main requirements of the antenna to be used for FM transmitters are : -
Wide-band usage from 88 to 108 MHz range. Omni-directional horizontal pattern of field strength. Circular polarization for better reception. High gain for both vertical and horizontal signals. Two degrees beam tilt below horizontal Sturdy design for maintenance-free service.
Further, depending on the type of tower available for mounting the requirement is for two types of antenna. The first type is to be mounted on a small cross-section AIR Tower. For which a pole type FM antenna has been selected. For mounting on the existing TV towers, a panel type antenna has been used. The cross section of the TV tower at the AIR aperture is 2.4 x 2.4 m. the pole type antenna is quite economical as compared to panel type antenna, but it can not be used on large area towers. For our requirement, the antennae supplied by M/s. SIRA have been found suitable.
Pole Type Antenna The pole type antenna is mounted on one of the four faces of the tower. This system will give a field pattern within a range of 3 dB. The antenna is mounted in such a direction in which it is required to enhance the signal. The important parameters for this antenna are : Weight VSWR Gain Rating of each dipole
200 Kg. (for 6 dipoles). 1.4 : 1 5 dB 5 kW
The other important features are :
Very low power radiation towards Transmitter building.
Spacing between dipoles is 2.6 m and all the dipoles are mounted above the other on the same face.
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Lengths of feed cables of dipoles will be different and has been calculated to give a beam tilt of 2o below horizontal.
The feed point of the antenna is looking towards ground so as to avoid deterioration of the insulating flange. This flange consists of high density PVC. The life of this is expected to be about 7 to 10 years.
The distance of the feeding strip is 240 mm from edge and this should not be disturbed. All the six dipoles are mounted on a 100 mm dia Pole. This pole is supported by the main tower.
The antenna is fed through a power divider which divides total power into 6 outlets for feeding the 6 dipoles. The power divider is mounted on a different face of the tower.
The main feeder cables, power divider branch feeder cables, and dipoles are of hollow construction to enable pressurization of the system.
The antenna can handle two channels with diplexing.
Suitable terminations are supplied for terminating the output of power divider in case of failure of any dipole.
Panel Type Antenna The panel type antenna is to be used on TV tower. Doordarshan have provided an aperture for FM antenna on their towers. The size of this section is 2.4 x 2.4 mtrs. and its height is different at different places. The antenna system envisaged for FM broadcasting consists of a total of 16 panels. For omni-directional pattern 4 panels are mounted on each side of the tower. Ladders for mounting these panels have already been provided on the four sides of the tower. Each panel consists of :
Reflector panel Two numbers of bent horizontal dipoles and Two numbers of vertical dipoles
The capacity of each dipole is 2.5 kW. Therefore, each panel is able to transmit 10 kW power. The reflector panels are constructed of GI bars whereas the dipoles are made out of steel tubes. Since each panel consists of 4 dipoles, there are a total of 64 dipoles for all the 16 panels. Therefore the power divider has 64 outlets to feed each of the dipoles. The power divider will be mounted inside the tower. This antenna gives an omni-directional pattern when the panels are mounted on all the four faces.
Feeder Cable
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Induction Course(Radio) For connecting the output power of the transmitter to the dipoles through the power divider, a 3” dia feeder cable has been used. This cable is of hollow type construction and has to be handled very carefully. From the building to the base of the tower, the cable is laid on horizontal cable tray. Along with the tower this is fixed on the cable rack provided for this purpose. The cable is clamped at every 1.5 m and the minimum radius of bending of this cable is about 1 m. The cable has been provided with two numbers of EIA flange connectors of 3 1/8” size on both ends. Both the connectors are of gas-stop type. The cable connector on the antenna end i.e. on top of the tower is made gas-through before hoisting. This is achieved by drilling a hole through the Teflon insulator inside the connector. A dummy hole (drilled only half way) is already provided by the manufacturer for this purpose. The weight of the cable is about 2.7 kg per meter and the power handling capacity is about 27 kW. Since enough safety margin has been provided in the power handling capacity, no standby cable has been provided. This cable can be used later for two transmitters by diplexing. The attenuation loss of the cable is about 0.44 dB per 100 meter length. The cable and the antenna system should be fed with dry air by means of a dehydrator provided with the transmitter.
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